a. Transceiver Range Extender
It will be appreciated that in marine or aviation use, portable battery-powered portable transceivers are limited both in their power and also by the height of the antenna above the terrain. Moreover the antennas for the portable units, being of a shortened variety, are inherently very inefficient. Because these units use self-contained batteries, the amount of power is limited so that full 25 watt operation is not possible. The above conspires to limit the range of the portable transceiver to reliable communications of only between three and six miles. On the other hand standard VHF marine transceivers with properly located and tuned antennas have a line of sight range of 25 miles. Of course, longer ranges are achievable with increased antenna height.
In an effort to extend the range of portable VHF marine transceivers, auxiliary antennas have been attached to the portable unit which are both cumbersome and often ineffective due to their temporary mounting and the low height. Due to the restrictions imposed by the FCC and also due to power available from NICAD batteries, the maximum transmit power is usually limited to six watts. Moreover at a six watt power level, battery lifetime is often no more than 20 minutes of transmission.
It should be noted that the portable unit is very popular because it provides convenience in that one can move around one's boat and still communicate albeit only over short distances. However, this short range limitation is a very frustrating problem to the boater in spite of the convenience of a portable unit.
By way of background, long range communication repeaters have been utilized to extend the range of portable transceivers. These systems in general are full duplex, split frequency systems, with a receiving channel at one frequency, and a transmitting channel at another frequency. Note that all users have equal access to a repeater and that in general they operate only on two assigned split channels. Thus repeaters can handle only one conversation at a time for the entire coverage area. They therefore do not operate in any manner similar to a VHF radio which is a simplex system and has as many as 182 assigned channels. It will also be appreciated that FCC regulations normally preclude the use of unattended repeaters or unattended transmitters for unrestricted multi-channel use.
The result of the inability to utilize repeaters in the marine portable service market is that boaters are denied the convenience of a portable or hands-free system capable of communicating over ranges normally associated with the conventional VHF base station.
Note further that numerous remotely-controlled base stations exist both in amateur and in land mobile services, in which transmitter keying is dependent upon the absence of a subaudible carrier to delineate the end of a received transmission. These services do not however experience the same problems of congestion and interference as do either the uncontrolled citizens band radio service or the marine band radio service. For commercial service, channels can be assigned as well as subaudible tone frequency to control transmit/receive cycles. The result is that simplex communication can be achieved with the utilization of the subaudible carrier tone to control transmit/receive cycles.
The problem with utilizing a subaudible tone system in the marine or citizens band environment is that the subaudible tone system will not work without assigning individual tones. Thus, in order for any simplex service to utilize subaudible tone control, other interfering transceivers must either not use subaudible tones, or be restricted to subaudible tones of a different frequency.
Moreover, absent a subaudible tone system, the interference mentioned hereinabove is so severe in the marine VHF bands, that the base station would detect an interfering station in the background which would prevent switching the base station to its transmit mode. This is because the base station would be transmitting the received interfering signal to the portable unit which would prevent the portable unit from transmitting a signal to the base station to change from a receive to a transmit mode. Thus the operator of the portable unit would have to wait for the interfering station to cease transmission before he could switch his base station to the transmit mode. In summary, on crowded channels, simplex transmission alone cannot be used to extend the range of portable units through the use of remotely-controlled base stations.
In contradistinction to the simplex systems described above or to the duplex systems noted hereinbefore, the Subject System is not a repeater system but rather involves the remote control of a base station through a hybrid duplex system and a very low power, limited range, inexpensive portable unit. Both AM and FM service is contemplated, with FM being preferred. For simplicity of explanation it will be assumed that the base station operates in the 156 and 170 megahertz VHF marine band, whereas the remote control system operates in the 49 megahertz band. The reason for the low power of the portable unit is not only one of cost, but also is one relating to limiting interference. The reason for the modified duplex operation is that, if pure simplex operation were to be desired, the transmission from the base control transceiver to the portable unit would desirably cease upon termination of the incoming signal on the channel. However a distant signal may continue to be received by the base station long past the time that the desired signal has ceased. This then would result in continued transmission to the portable transceiver despite the fact that the desired signal has ceased, which would block the ability to transmit a signal from the portable unit to the base control transceiver.
In order to prevent this occurrence, the base control transceiver must be able to receive a signal from the portable unit on a first frequency so as to change the base station from a receive to a transmit mode while a signal is being transmitted to the portable unit on a second frequency. This can only be done with a hybrid duplex arrangement in which the base control transceiver can simultaneously receive one of the split channel 49 MHz signals from the portable unit while transmitting the other split 49 MHz channel signal to the portable unit. It will be appreciated that the marine VHF radio is simplex system, whereas the communication to and from the portable unit is duplex. The term duplex refers to the capability of handling transmission and receipt of signals simultaneously on two different frequencies.
As part of the Subject System, the effective range of a portable or miniaturized battery-powered transceiver is extended through the use of the remotely-controlled base unit which is under the control of a low power drain base control transceiver that is always ON, in which the base control transceiver is powered by its own standby battery. Note that the base control transceiver can be built into the base station; or it can be a separate module. In any event, the standby battery is internal to a base station control transceiver. When a properly encoded identification (I.D.) or "authorization" code from the portable receiver is detected, the base station is turned on and connected to ship's power. From there on, the full power of the base station is available to the portable transceiver.
In operation, the base station transceiver is provided with the above-mentioned base control transceiver which communicates with the portable unit in the following manner. The base control transceiver receives and transmits respective split channel signals on f.sub.1 and f.sub.2 in the 49 MHz band. When the portable unit is first turned on, the "authorization" code is transmitted to the base control transceiver to activate the remote control system to allow reception by the portable unit of transmission on the marine channel designated by the channel selector of the portable unit. This channel is designated by a "channel select" code, followed by a "squelch" code to set the squelch of the base station. When it is desired to transmit from the portable unit, a push-to talk (PTT) button is depressed which again transmits the predetermined "authorization" or I.D. code on f.sub.1 to the base control transceiver. Until the portable unit is turned on, the base control transceiver is operating on a spare or NICAD rechargeable batteries, with the transceiver in its quiescent state drawing no more than a few milliamperes. Upon receipt of the authorization code when the portable unit is switched ON, the unattended base control transceiver automatically turns on the base station transceiver and thence proceeds to set the channel of the receiver to that dialed in at the portable unit. At this time the base station squelch may also be set. The transmission from the portable unit is demodulated at the base control transceiver and the audio recovered therefrom is - coupled to the base station transmitter along with the appropriate channel and a transmit or keying command to place the base station transceiver in the transmit mode. From this point the audio is transmitted by the base transmitter at full Power through the antenna normally coupled thereto.
Thus audio is transmitted from the portable unit, as well as the factory-preset "authorization" I.D. code. The I.D. code is sent every time the push-to-talk (PTT) switch is depressed to place the base unit in the transmit mode. A different "end of transmission" code is sent each time the PTT switch is released to place the base station in the receive mode. In one embodiment this is a subaudible, audible, or superaudible tone.
Once the base control transceiver has received and identified the appropriate authorization code from the portable unit, in one embodiment the entire remote control system is locked on, absent time-out timers, until an "end of use" code, is sent from the portable unit. This "end of use" code may be generated in response to turning of the ON/OFF switch of the portable unit to its OFF position, and precludes the necessity of physically turning off the base unit. Thus the system may be provided with a separate "end of use" code, so that the base station can be remotely shut off with the turn off of the portable unit.
More importantly for the unattended portion of the system, the base unit is turned off by a timer a predetermined time after the I.D. code is sent, so that, for instance, the base unit will be disabled fifteen minutes after the last I.D. code, this turns the base station off and is provided so that no unauthorized RF transmission on f.sub.2 will occur, both to save batteries and for any unforseen activation of the system which would result in useless transmission on f.sub.2 from the base control transceiver. This timer also disables any unnecessary power-draining circuitry in the base control transceiver which does not affect its standby receive mode. The above timer can be defeated by a separate switch if longer use is required, such as all-day monitoring of a channel. If this is the case the "end of use" code sent when the portable unit is turned off prevents battery run down and further 49 MHz transmission.
As a further safeguard, in one embodiment a second timer disables the base station transmit mode, for instance, two minutes after an "authorization" code is received. The purpose is to counter any loss of control of the base station transmitter so that no unattended RF transmission from the base station can exceed the preestablished time period. If a transmission lasts more than two minutes, it is presumed that control over the base unit is lost and all transmission from the base station is shut down by returning it to the receive mode. It will be appreciated that the "end of transmission" code causes the base station to go into the receive mode even if a carrier is present on the 49 MHz band, thus preventing other portables from hanging up the system.
More particularly, in the receive mode the base control receiver receives the incoming signal from the portable unit, demodulates it, and provides both recovered audio to the base station transceiver and a "carrier present" signal to activate the base station transmitter. The base station transmitter is operated only when all four conditions are present: i.e., a valid ID code has been received; an end of transmission has not been received; the carrier present is high; and, the short duration timer has not timed out or has been reset. The base control transceiver in turn transmits the recovered audio at 49 megahertz on f.sub.2 to the portable unit, eg. on one half of the split channel originally designated by the portable unit. The portable unit then reproduces the audio at the remote location.
As a feature of the subject system, a further safeguard for the unattended portion of the system may include a mandatory brief receive cycle prior to any initial base station transmission so that the operator can ascertain if his chosen channel is free. This can be accomplished by use of a base station-ON tone transmitted to the portable unit once the portable unit makes contact with the base station. If there is traffic on the channel it will be heard after the tone.
As a secondary feature, both 49 MHz units may be provided with a clear channel search mode when the portable unit is first turned on. For FM systems, circuits within one unit scans the available channels for the one with minimum signal. Once this occurs, both units are set to this channel.
Alternatively, one can search for a clear channel by measuring relative signal strengths and choosing the channel with the least relative signal strength indicator (RSSI) output. This improves the duplex communication, and is a system which can also be actuated at any time from the portable transceiver by turning it OFF and ON.
What will be appreciated is that with the remote control of the base station, a low power, 1/10 watt, portable or even hands-free operation can be achieved so that the operator of a vessel can walk around his boat and communicate through the base station without any hard wire connection to the base unit. Moreover, should a person be on a beach or in a dingy, he can access the base station, within a thousand feet, and thereby utilize the facilities of the remotely controlled base station to effectuate long distance communication. This can be done even before the operator arrives at his vessel. Because of the low power of the portable unit, interference is kept within acceptable limits. Of course if full 6 watt power is desired, the portable unit may be provided with a full up VHF transceiver capability in addition to its remote control function.
Note that while VHF service is discussed herein, the Subject System may be used in any service such as SSB, citizen's band, or the amateur band if remote control of an unattended multi-channel base station is desired.
In summary, a combined simplex/duplex remote control system is provided to extend the range of a portable transceiver and to permit the operator of the portable unit to communicate hands-free through the utilization of his own full power base station and antenna. Here a modified full duplex system is used to control transmit and receive cycles from the portable unit so that the system can operate regardless of interference on the channel. This approach provides for a universal system in which other standard transceivers can be communicated with without providing them with subaudible tone capability or in any way modifying them. In one embodiment the base control transceiver is provided with a rechargeable battery and is left on continuously, with the base control transceiver being directly coupled to the vessel s battery so that the base unit can be turned off until such time as the appropriate I.D. code is received from the portable unit, at which point the base station is connected to battery power. Thus the base station can be left unattended in a low power mode until such time as it is accessed by its corresponding portable transceiver which sends the appropriate I.D. code. The base station can therefore be operated both in its normal mode or remotely when accessed through the transmission of the particular I.D. code provided by the portable unit.
As a result, the base station can be remotely controlled by a very low power, limited-range, portable unit to extend the nominal 3 mile range of the portable unit to the full operating range of the base station. The range extension is a result of the increased base station power, as well as its increased antenna height and better antenna efficiency.
Safeguards for unattended use include "authorization" codes, "end of transmission" codes, "end of use" codes and two timers for time-out a first relatively long predetermined time after "authorization" code receipt to shut down the base station and all RF transmitting systems; and a second relatively short predetermined time after the beginning of a transmission as evidenced by "authorization" code receipt to return the base station to its receive mode, thereby to solve the problem of loss of base station control.
As another feature, the portable unit may also be used to transmit an "end of transmission" tone burst to the base control transceiver on whichever split frequency is not in use. This assures that an adjacent close transmitter will not interfere with "end of transmission" tones from the portable unit, and permits returning the base station to the receive mode, even if there are interfering 49 MHz signals in the area.
Thus the quasi-duplex system is invoked only when there is congestion on the frequency that the base station is tuned to; and also if there is interference on the remote signalling frequency This prevents the system from locking on to a spurious or undesired interfering signal.